z420 and evo 960 Nvme, non boot configuration

Hi all,

I have a z420 with Windows 10, I put in PCi expres slot an NVME 960 evo with adapter. This is a non bootable disk, all god but not happy with the speed wich are actualy lower than 950 evo SATA.

My question is this actualy the desing limit, I've seen other talking about 1-2 GB data transfer, I have in this configuration 400 MB (read). I use this one to store VM's

Configuration is 

2x Evo 950 on sata III where Windows 10 1809 is instaled (RAID 0)

1 x Evo 960 pci adapter 

Thanks